Hydraulic fracturing of oil- and gas-bearing shale formations for improving hydrocarbon production is an effective method to get more “trapped” oil and gas out of the subsurface, in which fracturing fluid (fresh water, brine, etc.) containing a number of components, including proppant, is injected via high-pressure pumps to increase the permeability of formation. The pressure generated on the surface decreases significantly when the fracturing fluid reaches subsurface fracturing zones, primarily due to the turbulent tendency of fracturing flow under very high Reynolds-number non-Newtonian flowing conditions. Addition of natural or synthetic polymers into the fracturing fluid is a practical way to reduce the pressure drop, also known as the “friction reduction.” The polymer additives serving for the purpose of friction reduction in an aqueous-based well stimulation operation are referred to as “fraction reducers”, whereas those used in the oil-based pipeline operations are commonly referred to as “drag reducers”. The requirement for friction reduction in hydraulic fracturing of oil- and gas-bearing formations is very different than use of friction reducers in transport of oil though pipelines that carry oil over long distances, in which the oil passes and is pumped through various substations. In hydraulic fracturing operations, the fluid must be pumped down into the formation very quickly and under high pressure so that the proppants are forced into fissures in the formation to physically hold open the fissures so that petroleum may be allowed to more readily flow out of the formation. In these operations, the friction reduction must occur very quickly and typically large volumes of water or brine must be prepared. In contrast, in the field of moving oil though pipelines, friction reducers are added directly to oil to reduce the friction of the oil as it moves through the pipelines for many hours and as the oil passes repeatedly through pumping stations to keep the oil moving. The friction reducers must optimally be able to withstand the repeated action of pumping stations and degradation over time.
Polymer-based friction reducers are composed of high-molecular weight, water-soluble polymers. Examples of these polymers include polyacrylamide, hydrolyzed polyacrylamide, polyacrylamide/acrylate copolymers or those based on such a backbone or on poly(meth)acrylamide and poly(meth)acrylic acid backbones, polyethylene oxide and derivatives thereof, and semi-synthetic polymers such as carboxymethyl cellulose or hydroxy-ethyl cellulose, or the natural-occurring biopolymers such as guar gum.
It has long been recognized that water-soluble friction reducers are easily degraded when exposed to water under high-temperature, high-pressure, high-salinity and/or high-shear conditions. Along with numerous laboratory and field studies to improve the stability of polymers in solution, Ma et al. also conducted systematic theoretical studies based on quantum chemistry computational modeling to insights to of polymer hydrolysis and stability based on energetics calculations. See Ma, et al. in El Sevier Polymer Degradation and Stability, 121 (2015) 69-77, Theoretical studies of hydrolysis and stability of polyacrylamide polymers.) To preserve shelf life and for storage purposes prior to use, direct contact of friction reducers with water should be avoided. Several methods can be used to accomplish this. One method is to keep the friction reducer in the dry-powder form. That is, after being synthesized, polymers are separated from solution, dried, purified and often ground into powdered products. Products made by this approach can be referred to as “dry friction reducer”, or “DFR”. Another approach is to synthesize polymers by inverse emulsion polymerization (oil-external), preserving the water-soluble polymer inside an oil phase. Because a majority of friction reducers are polyacrylamide (PAM), products made by this approach are often referred to as “emulsion PAM-based friction reducers”, or “ePAM” for short.
Friction reduction achieves its maximum effect when the friction reducing polymer is fully extended in the aqueous solution, but starts to decrease when the polymer degrades. The stability requirements of a polymer depend on the type of field operation. For example, in polymer-augmented waterflooding enhanced oil recovery (EOR) operations, water-soluble polymers often are required to be stable under high-temperature, high-pressure, or high-salinity conditions for several months. Whereas in hydraulic fracturing operations, a major cause of polymer degradation is ultra-high shearing stemming from very high pumping rates, and the required time period for stability of a friction reducer in such as use is often only one or two hours. Another important timing criterion for friction reduction in hydraulic fracturing operations is how fast the polymer can achieve its maximum functionality. For an ePAM product, this is often referred to as “inversion time”, that is, the time it takes to break the emulsion for release of the polymers when the ePAM dispersion of suspension is diluted in water. For a DFR product, inversion time is related to the time it takes to for the polymer to be hydrated by water.
The inversion and/or hydration time for of a friction reducer in typical hydraulic fracturing operations often is required to be less than 1 minute. This has imposed a great challenge to the field application of many product candidates, especially because the inversion and/or hydration time is also influenced by a number of field conditions such as temperature and the type and concentration of dissolved salts in the fluid. Fast inversion times (less than 30 seconds) by ePAM can be achieved under fresh/tap water conditions when the active polymer content is limited to 30 weight percentage or less. Additional additives such as surfactants have been used to shorten the inversion time. However, additional material and logistical costs as well as environmental issues of including additional chemicals in fracturing fluids have been a great concern for the ePAM products.
Hydration of solid powder polymers can be challenging for use in fracking operations as they tend not to dissolve quickly enough. U.S. Pat. No. 9,334,438 provides a good overview of the hydration problem and how current oil service industries have dealt with this issue. Pre-hydration of the polymer powder in a mixing unit is often required. A variety of mechanical devices have been disclosed in previous work. Examples include, but not limited to, U.S. Pat. Nos. 5,190,374, 5,382,411, 5,426,137, 5,947,596, and 5,981,446, as well as more complex commercial units such as the precision continuous mixer or programmable optimum density blender disclosed in U.S. Pat. No. 9,334,438. In U.S. Pat. Publ. 2012/0157356, Dawson et al. also disclose conceptual designs for batch mixing in a continuous process blending equipment.
For cost-effective application of DFR in hydraulic fracturing operations or stimulation operations, what is needed is the direct feeding scheme of DFR into the fracturing fluids without the prior art pre-hydration step, as illustrated in FIG. 1, a procedure that is often referred to as addition “on-the-fly”, which is currently only available for select ePAM products. When using a dry powder product, the most difficult hurdle to overcome is achieving ultra-fast hydration of DFR in an aqueous fluid, ideally in less than 1 minute, in water, without, at the same time, suffering the formation of so-called “fisheyes”, as described further below.
Fundamental flow rheology theory teaches that the hydration time of dry powders is proportional to their active water contact surface area, and that the reduction of the particle size can affect the hydration rate. U.S. Pat. No. 3,730,275 discloses that by including various sizes of particles, the shear degradation rate of friction reducers can be modified. In particular, U.S. Pat. No. 3,730,275 discloses a dry powder with preferred particle sizes as follows: At least 20% by weight within a size range of 5 to 30 mesh (or 4000˜595 microns), at least 20% by weight within a size range of 40 to 90 mesh (or 420˜163 microns), at least 20% by weight within a size range of 100 to 200 mesh (or 149˜74 microns), and with any remaining powder of 0% to 40% being in a size range of 5 to 300 mesh (or 4000˜74 microns). The purpose for this prior art disclosure is to provide friction reduction yet withstand shear degradation of polymer. Thus at least 20% by weight of the particles are greater than 30-mesh (˜600 microns), while particles smaller than 40-mesh account for at most 80% by weight. The particle size range is very broad, that is, from about 74 to 4000 microns. The '275 reference does not disclose or discuss any hydration times by its polymer mixtures. (The term “micron” herein and throughout is equivalent to the SI-recommended “micrometer,” abbreviated “μm.”)
Attempts to simply reduce the particle size of friction reducers do not necessarily result in the improvement of the shortening hydration time. Indeed, it has been found that particles with too fine a size tend to aggregate or agglomerate upon contact with water to form globules or the so-called “fisheyes”, which are very difficult to (re-)hydrate. U.S. Pat. Nos. 5,849,862 and 6,642,351 disclose that the agglomerated forms of precipitates of smaller particles dissolve more slowly than those of larger particles. U.S. Pat. No. 3,839,500 discloses that in order to have very few lumps during dissolution and have the powder well dispersed, the dry powder mixture should have less than about 5% by weight of particles of a size smaller than 44 mesh (˜370 microns), with most particles between 16 and 44 mesh (or 1190˜370 microns), and the polymer powders are made from “various block polymers of mixtures of ethylene and propylene oxides”.
Notwithstanding disclosures of particle sizes of polymer dry powders influencing their dispersibility or degradation t, it has been unexpectedly discovered that a very short hydration time, viz. about 1 minute or less, and near complete dispersibility leading to significant maximum friction reduction efficiency can be achieved simultaneously using a dry powder polymer, rendering direct injection of DFR possible. The direct or on-the-fly injection of DFR has been believed to be very difficult if not impossible to achieve in well stimulation operations, such that ePAM remains the major friction reducer type in current markets.